This invention relates to novel trifunctional blocked isocyanates containing an isocyanurate ring and to compositions containing same. More particularly, the invention relates to both solvent based and aqueous coating compositions including the novel trifunctional blocked isocyanate compound as a crosslinking agent. Still more particularly, the invention relates to such compositions which are capable of being exposed to higher temperatures without resulting in premature crosslinking, thus resulting in compositions with a better shelf stability and more desirable high temperature curing characteristics.
It is known to prepare isocyanurate containing isocyanates by cyclotrimerization of difunctional isocyanates. See for example A. A. S. Sayigh, Advances in Urethane Science and Technology, Volume 3 (1974), p. 141 and S. A. Fischerand W. H. Snyder, Organic Coatings and Plastics Chemistry, Volume 45 (1981), p. 36. Such a cyclotrimerization of difunctional isocyanates occurs in accordance with the following equation: ##STR1## Where I is an isocyanurate ring. Usually the reaction does not stop at the stage where there exists a pure trifunctional product as above, but continues through the formation of polyfunctional oligomers of the following type: ##STR2##
In view of the above, in order to obtain a pure trifunctional material it would be necessary to interupt the reaction at certain conversion levels and deactivate the catalyst employed. This is extremely difficult, if not impossible, to achieve and results at best in a mixture of the pure trifunctional material along with other oligomeric type reaction products.
A still further difficulty with the above method of preparing isocyanurate ring containing compounds is encountered in those cases where the diisocyanates to be trimerized are those in which one of the isocyanate groups has a reactivity greater than the other. Obviously, if the material is to be employed as a crosslinking agent, it is desirable to have the most reactive isocyanate groups available for the crosslinking reaction. However, when trimerizing this type of material, it trimerizes through the most reactive isocyanate group, thus resulting in a isocyanate functional product of significantly lower reactivity inasmuch as the isocyanates of lesser reactivity are those remaining after the trimerization. Thus, for example when toluene diisocyanate is trimerized in accordance with the teachings of the prior art, as depicted in the following formula, the isocyanate group in the para position reacts during the trimerization reaction, thus leaving the ortho isocyanate group available for the crosslinking reaction. ##STR3##
It has been found that by utilizing a blocking reaction and a subsequent cyclotrimerization reaction, that it is possible to prepare pure trifunctional polyisocyanate crosslinking agents containing isocyanurate ring.